This invention relates to the measurement of physiological signals, and, more particularly, to obtaining a time-series heart signal by magnetocardiography.
The heart is a muscle that operates in response to a variety of electrical signals originating both inside and outside the heart. The electrical signals may be monitored and displayed, producing a time-series of heart signals. These cardiac cycle signals can, in turn, be used to assess the health of the heart and the presence of some types of heart problems. The understanding of the functioning of the heart in health and in sickness may lie, in part, in determining the temporal and spatial relationships of various: sources of the measured electrical signals to each other.
Most studies of the heart and its electrical signals have been performed by electrocardiography (ECG). Electrical pickup sensors are attached to the surface of the body. The electrical signals produced during the functioning of the heart are detected and recorded, and are available for more detailed studies at a later time. The ECG signals show characteristic waveform shapes and patterns which physicians use to identify normal and abnormal heart function. In particular, there are regular intervals in the ECG signal pattern during which the ECG signal pattern is nearly zero, known as isoelectric intervals.
Magnetocardiography (MCG) provides an approach for detecting both the time series signal output and the locations of the sources of the electrical cardiac signals. This technique is based upon the fact that a magnetic field is generated by the electrical currents flowing in the heart. The magnetic field is detected by a sensitive magnetic field sensor, preferably an array of such sensors provided in a biomagnetometer, which is placed outside the body of a subject. The output of the sensors is amplified and filtered and made available for analysis.
In addition to the magnetic fields produced by the heart, the magnetic field sensors detect environmental magnetic fields which constitute noise that interferes with the measurement of the heart signals. Such environmental magnetic fields include the earth's magnetic field, magnetic fields produced by nearby electrical apparatus, and magnetic fields produced by completely unrelated equipment. The magnetic field produced by the heart is quite small compared to these environmental magnetic fields, and the cardiac signal can therefore be lost in the noise of the environmental magnetic fields.
It is therefore necessary to separate the cardiac magnetic field from the environmental magnetic fields with which it is mixed. One approach is to place the subject in a magnetically shielded room (MSR) that reduces the magnitude of the environmental magnetic fields that reach the sensors. Such MSRs are expensive, and the need to use such MSRs limits the use of magnetocardiography to those locations which have them. A second approach is to detect the noise component, and then seek to remove it by filtering. Conventional high-pass and band-pass analog and digital filters have been utilized for this purpose in the past. These techniques, which produce a filtered signal, have the shortcoming that they produce an output cardiac signal that is distorted from its true time-series values as a result of the filtering methodology. That is, the filtering removes not only the interference, but also components of the cardiac signal that lie in the same range of frequencies. The filtered, but distorted, signal may be used for some purposes, but for other purposes the distortions render the signal of significantly less value. In addition, digital filtering, the preferred filtering approach, consumes a significant amount of computer time when there are a large number of sensors whose outputs are to be processed. The signal analysis may therefore not be possible in real time, a drawback for some applications.
There is therefore a need for an improved approach to the study of the heart in which a true, undistorted time-series heart signal can be measured by magnetocardiography. Although measurements of heart signals are presently of most interest, the same improved approach may find application in relation to other physiological measurements. The present invention provides a necessary advance in the art toward fulfilling this need, and further provides related advantages.